Demand for hydrogen is expected to increase rapidly in all industrial fields. Within this context, hydrogen sensors for detecting low concentration hydrogen gas leaks or for measuring the concentration of hydrogen gas are under development.
Various sensors for detecting hydrogen gas has been proposed in the past. For example, there has been a proposal (Patent Literature 1) for a sensor that takes a rare earth metal thin film such as yttrium (Y) or lanthanum (La) as a hydrogen detection film.
This sensor utilizes the fact that when the rare earth metal composing the detection film is exposed to hydrogen, the physical properties change. In this proposal, the physical change in this detection film is detected, and that change is utilized to detect hydrogen. The rare earth metal composing the detection film is adversely influenced by non-hydrogen components such as nitrogen, oxygen, ammonia, or hydrocarbons present together with the hydrogen. This sensor covers the surface of the rare earth metal film with a hydrogen permeable protective film composed of hydrogen permeable palladium (Pd), platinum (Pt) or an alloy of these in order to avoid the adverse influence of the non-hydrogen components.
Nonetheless, the volume of the hydrogen permeable metals such as Pd and Pt repeatedly expand and contract when absorbing and releasing hydrogen. The mechanical stress induced by this expansion and contraction causes deterioration and cracking of the hydrogen permeable metal film over time. For that reason, hydrogen sensors using a hydrogen permeable metal film as a protective layer have poor durability.
The hydrogen permeable metals like Pd and Pt are prone to diffuse into rare earth metals such as yttrium (Y) and Lanthanum (La). When this diffusion occurs, the hydrogen detection capacity of the detection film composed of rare earth metals decreases over time. Consequently, hydrogen sensors using the aforementioned hydrogen permeable metals in the protective film have durability issues.
Moreover, because Pd and Pt are extremely expensive metals, hydrogen sensors that use these metals in the protective film have high manufacturing costs.
In order to resolve the aforementioned problems, a hydrogen sensor using a protective film in which hydrogen permeable metal particles are dispersed in a ceramic material was proposed (Patent Literature 2). This protective film has hydrogen permeable palladium (Pd), platinum (Pt), niobium (Nb), vanadium (V), tantalum (Ta) particles or alloy particles of these dispersed into a ceramic composed of nitrides or oxides of aluminum (Al) and/or silicon (Si), or of silicides of rare earth elements.
In this hydrogen sensor as well, volume changes of the detection film in association with the lattice state when hydrogenating and dehydrogenating cause repeated stress to the protective film. As a result, the protective film may be damaged. In order to prevent damage to this protective film, the thicknesses of the detection film and the protective film are adjusted. However, this method restricts the degrees of freedom in terms of designing the sensor specifications. For example, if the thickness of the detection film is increased, it is necessary to make the protective film thicker as well. In this case however, the problem emerges that the sensor response speed is slowed.
Further, oxygen and water vapor may permeate the protective film through pinholes that may be present in the protective film or through film defects caused by impurities included in the protective film. In such a case, these harmful non-hydrogen gases alter the detection film, and it is necessary to make a thick protective film in order to prevent alteration.
Lamination of 2 layers of protective film onto the detection film was proposed in Patent Literature 2 as a method to resolve this problem. This method is effective for resolving the aforementioned problems. However, the problem of increased manufacturing steps then arises.
For these reasons, it is desirable to develop a low cost hydrogen sensor with excellent durability.    Patent Literature 1: National Publication of Translated Version No. 2002-535651 (Claims)    Patent Literature 2: Japanese Patent Application Laid-open No. 2005-274559 (Claims)